Operating inverters with a resonant circuit is attractive since sinusoidal currents and voltages can be achieved, and since inverter switching devices can operate with low switching losses. Another advantage of using resonant inverters is the potential for obtaining reduced electromagnetic interference due to the low harmonic content of the sinusoidal current and voltages.
Such inverters generally require a stress-relieving circuit, or snubber, connected across each of the power semiconductor switching devices. Snubbers function to: control the rate-of-rise of voltage and current; reduce switching losses of the power semiconductor devices; suppress overvoltages; abate noise and electromagnetic interference; and avoid secondary breakdown of the power semiconductor devices.
For inverters operating with a load having a leading power factor, such as thyristor inverters, both turn-on and turn-off snubbers are generally required. However, when the load on the inverter is running at a lagging power factor, device current waveforms for inverters comprised of transistors are more retarded in phase relative to the output voltage of the inverter than those of thyristor inverters. A lagging power factor is ensured for series resonant operation above the resonant frequency. Under normal load conditions at a lagging power factor, current is transferred from each switching device to its respective anti-parallel feedback diode and/or integral parasitic feedback diode prior to turning on the switching device. Hence, for such operating conditions, there are no turn-on switching losses in the power semiconductor switches. However, there are turn-off switching losses which can be reduced by connecting a simple snubber capacitor directly across each switching device. Since no energy is stored in the shunt capacitive snubber after the power semiconductor switch in parallel therewith has switched on, there is no need for a current limiting resistor or a parallel-connected diode-resistor combination in series with each capacitor. The result is lossless snubber action and, therefore, low semiconductor switch power dissipation and low voltage overshoots at device turn-off.
In series resonant circuit operation, there is a switch commutation delay between switching the circuitry in diagonally opposed corners of a bridge inverter. The lossless snubber capacitors limit the rise and fall times of the typical square wave voltage driving the resonant tank circuit. Disadvantageously, however, under no-load or light-load conditions, the available load current is generally insufficient to "reset", or discharge, the snubber capacitors during the power switch commutation delay, and lossless snubber action is compromised.